The following is not an admission that anything discussed below is citable as prior art or part of the common general knowledge.
Polymeric separation membranes in the form of small capillary tubes or hollow fibers can be made from a variety of polymers by different methods including NIPS (non-solvent induced phase separation) and TIPS (thermally induced phase separation). Examples of NIPS processes are described in U.S. Pat. Nos. 3,615,024, 5,066,401 and 6,024,872. Examples of TIPS processes are described in U.S. Pat. Nos. 4,702,836 and 7,247,238. The membranes may have a separation layer on the inside or outside and may be used, for example, for microfiltration (MF) or ultrafiltration (UF).
A benefit of membranes in water treatment is their ability to remove bacteria from water, effectively providing physical disinfection. However, it is important to maintain mechanical integrity of the membrane for its expected service life. With hollow fiber membrane modules, one mechanical failure mechanism is fiber breakage (often near a potting interface) as a result of fatigue.
International Publication Number WO 03/097221 A1 to Yoon et al. and U.S. Publication Number US 2002/0046970 A1 to Murase et al. describe embedding mono or multi-filament yarns longitudinally within the wall of a hollow fiber membrane as a way of reinforcing the membrane. However, upon flexing and movement of the hollow fiber, the longitudinal filaments are likely to saw through the softer membrane material and thus create a new failure mode. The inventors are not aware of any use of such a membrane in industry.
Another type of reinforced hollow fiber membrane that is currently used in industry uses a hollow textile braided sleeve coated or impregnated with a polymeric membrane. The braid provides the strength that is needed in MF/UF applications such as filtration of water suspensions or mixed liquor where continuous or intermittent agitation (with air or otherwise) of the hollow fibers is used to prevent fouling or accumulation of solids on the membrane surface.
Examples of braid-supported filtration membranes include U.S. Pat. No. 4,061,861 to Hayano et al. where a polymer is impregnated into a hollow braid to prevent shrinkage when operating at high temperature; U.S. Pat. Nos. 5,472,607 and 6,354,444 to Mahendran et al.; U.S. Pat. No. 7,267,872 to Lee et al. where the membrane is coated on the outside surface of the braid and penetration is limited; and, U.S. Pat. No. 7,306,105 to Shinada et al. where the braid is coated with two different porous layers.
Braid-supported hollow fiber membranes are normally prepared as follows. The braid is fabricated on a braider, wound on a bobbin, repackaged to larger spools by splicing ends together, and then transferred to a spin line where it is unwound and then coated or impregnated with a polymer solution in a coating head. Relatively thick walled and tightly woven braids are used so that the braid will be round-stable, meaning that it does not flatten out through winding and unwinding and is still round when inserted into the coating head.
Braided supports thus have some disadvantages. For example, round-stable braids are fabricated on braiding machines with a large number (for example 16 or more) of braiding carriers. Each carrier is supplied from a different bobbin and the bobbins must cross paths in the braiding machine. The bobbins must accelerate, decelerate and reverse radially every time the carriers cross each other. This is a costly and slow operation. Small diameter braids (less than 2 mm) are normally made at a speed of less than 0.5 m/min. In contrast, the braid coating or impregnation operation is typically done much faster, for example at a speed of greater than 15 m/min, thus the need for separate operations with a spool transfer step in between. Unwinding a large spool of braid at constant tension for membrane coating is also challenging, and the coating process must stop from time to time to change spools.
In addition, the braids used for membrane support are typically of a relatively large diameter (>1.5 mm). This is because braiding speed and braid costs are generally diameter independent, but the surface area increases proportionally with diameter. Braids thus normally have a large diameter as well as a thick wall, required to make them round-stable. As a result, the ratio of inside-to-outside diameters is small, typically smaller than 0.5. This is the normalized parameter that determines the pressure loss to conduct permeate through the lumen. High lumen pressure drop in thick wall braids limits the useful length of hollow fibers that can be potted in a module.
Fiber diameter is also a significant hidden contributor to overall membrane cost because the volume of a fiber is proportional to the square of its diameter, while the developed surface area is proportion to diameter directly. Therefore, at constant packing density of hollow fibers in a module and constant ratio of inside-to-outside diameters, an increase in the outside diameter of a fiber decreases specific surface area (area per unit volume) and increases specific polymer use (mass of polymer per unit surface area), both of which increase the cost of a membrane system designed to filter a given flow of water.
Introduction
The following is intended to introduce the reader to the detailed description to follow and not to limit or define the claims.
In the detailed description, various methods of making a reinforced membrane, devices for making the membranes, and the resulting membranes are described. The methods typically provide a reinforcing structure (sometimes called a “tubular cage” or “cage” herein) that includes filaments extending around the circumference of the membrane but without the filaments being part of a braided or woven structure. Some of the reinforcing structures also include longitudinal filaments. The methods and devices can be used to make a supporting structure in line with membrane formation steps, and also allow for a reinforced membrane to be produced that has a ratio of inside-to-outside diameters of 0.5 or more.
One method of making a reinforced hollow fiber membrane uses composite yarns. The yarns comprise generally continuous longitudinal filaments extending along the length of the yarn and other filaments having loose ends or loops, or both, that protrude from the longitudinal filaments. A reinforcing structure comprising the yarns is formed around the outer surface of a core, such as a mandrel, needle or fiber, with an outside diameter similar to the intended inside diameter or the membrane being made. In the reinforcing structure, the generally continuous longitudinal filaments are spaced around a circumference of the core, and are generally aligned with the length of the core. The ends or loops of the yarns extend around a portion of the circumference of the core and overlap or intersect with one or more filaments of one or more of the other yarns. A liquid membrane dope is applied to the reinforcing structure in a coating head (sometimes called a “spinneret” herein) and then treated to form a solid reinforced membrane. Optionally, the supporting structure may be relatively open compared to a braided support with the membrane dope fully impregnating the yarns. A separation layer may be located on the inside or outside of the membrane.
In the method mentioned above, or in other methods described herein in which a reinforcing structure is made over a core, the core may be fixed or movable. If the core is fixed, yarns or other filaments slide along and eventually off of the core. A fixed core may have an interior bore through which a bore fluid is injected through the coating head to help form the inside surface of the membrane. If the core is movable, the core moves with the yarns or other filaments through the membrane coating head or spinneret. A moving core may comprise a previously formed membrane wall or a soluble core that will be dissolved out of the membrane later.
The reinforcing structure and the membrane wall are preferably formed concurrently, though sequentially. For example, in one method of making a reinforced fiber using a fixed core, composite yarns are pulled along a mandrel and through a membrane coating head. Filaments of the composite yarns may be entangled with each other as the yarns move along the mandrel upstream of the spinneret, for example by a spinning device. A membrane dope flows through the coating head and around the yarns as they pass through the coating head. The filaments and dope leaving the coating head continue to a bath wherein the membrane dope forms a solid membrane wall.
Optionally, the filaments of the reinforcing structure may be bonded to each other at points of contact where they intersect. This may be done in a bonding device upstream of the membrane coating spinneret, for example by applying heat or UV light to the reinforcing structure. Alternatively, the bonding may be done in the membrane dope by way of solvents in the dope softening or solvent bonding the reinforcing filaments. Some or all of the filaments may be composite filaments having a component adapted to the bonding method.
In the coating head, the reinforcing structure passes through an annular passage around the core thus placing the reinforcing structure with the membrane wall. Optionally, the filaments of the reinforcing structure may also be smoothed in a die before they pass through a coating head.
One apparatus for making a hollow fiber membrane described herein comprises a mandrel, a creel at one end of the mandrel to distribute a plurality of yarns around the outer surface of the mandrel, a membrane dope coating head at another end of the mandrel, and an air spinning or vacuum spinning device located around the mandrel between the creel and the mandrel. One hollow fiber membrane described herein comprises a selectively permeable wall, a plurality of yarns attached to the wall and extending along the length of the membrane, and filament ends or loops of the plurality of yarns intersecting filaments of adjacent yarns.
Other methods, devices and membranes are also described herein. For example, some methods of making a reinforced hollow fiber membrane include steps of forming a reinforcing structure around the outside of the core, applying a liquid membrane dope to the reinforcing structure, treating the liquid membrane dope to form a solid membrane and dissolving the core. Other methods of making a reinforced hollow fiber membrane involve forming a reinforcing structure around the outside of a membrane wall acting as a core and bonding the reinforcing structure to the outside of the hollow fiber membrane. The membrane may have an internal or external separation layer, or a further separation layer may be applied over the reinforcing structure. Other methods of making reinforcing structures include spiral-wrapping filaments around a core and forming a non-woven fabric around the core, optionally on top of a set of longitudinal filaments. Corresponding membrane making devices and resulting membrane structures are also described.